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414
CHEMICAL ENGINEERING
bulb of the instrument or is lost by conduction to the air by way of the heavy metal parts. There are no published reliable investigations giving data in regard to the magnitude of the influence of all the factors noted above.
If industrial thermometers are used to indicate only changes in the temperature of a process, and not to give actual temperatures, it may not be necessary to consider the various sources of error enumerated, but it should be emphasized that these instruments cannot be depended upon to give reliable indications of true temperatures, unless they are calibrated or standardized under the same condition as that of use, and after being put in use these conditions are not changed.
Emergent Stem Correction for Mercury-in-glass Thermometers.—A large proportion of the thermometers now sold are graduated and standardized for the condition of total immersion, that is, for the condition that the bulb and all the portions of the stem which contain mercury, are heated to a uniform temperature.
The thermometers are, however, rarely used under this condition, but in nearly all cases are used at partial immersion, the bulb and a short length of the stem being exposed to the temperature to be measured, while the remainder of the stem projects above the bath into the (usually) cooler air of the room. In the following it will be assumed that the thermometer is in a heated bath. In this case, the emergent mercury column is cooler than the bath, consequently the thermometer reads too low. The amount, in degrees, by which it reads too low, may be calculated from the formula,
S = an(T - 0
where S is the correction to be applied to the reading, a is the relative expansion of mercury in glass (0.00016 for centigrade mercurial thermometers and 0.00009 for Fahrenheit thermometers), n is the number of degrees of the mercury column emergent from the bath, T is the temperature of the bulb or bath, and t is the mean temperature of the emergent column. It may be noted that if t is a higher temperature than T, the sign of the correction will be negative, so that, if due attention is given to the sign, the formula holds whether the bath is warmer or colder than the emergent stem.
TABLE 4.—STEM COERECTION DATA FOR CLEVELAND OPEN-CUP FLASH AND FIRE-POINT TESTER Thermometer range —20 to 760°F. in 2° intervals, length about 15 in.
Temperature of bulb, degrees Fahrenheit	Degrees of mercury column emergent, degrees Fahrenheit	Mean temperature of emergent mercury column, degrees Fahrenheit	Stem correction, degrees Fahrenheit
208	200	173	0.5
308	300	174	3.5
408	400	169	8.5
508	500	172	15.0
608	600	180	23.0
Of the factors in the formula, the value of a has been given, n can be observed while T is (except for the error due to the emergent stem) equal to the reading of the thermometer. The value of t is the most difficult to determine, but an approx-